linux-ext4 - Re: The meaning of data=ordered as it relates to delayed allocation

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Date:	Mon, 19 Jan 2009 10:13:45 +0530
From:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:	"Theodore Ts'o" <tytso@....edu>
Cc:	linux-ext4@...r.kernel.org
Subject: Re: The meaning of data=ordered as it relates to delayed allocation

On Sun, Jan 18, 2009 at 07:52:10PM -0500, Theodore Ts'o wrote:
> 
> An Ubuntu user recently complained about a large number of recently
> updated files which were zero-length after an crash.  I started looking
> more closely at that, and it's because we have an interesting
> interpretation of data=ordered.  It applies for blocks which are already
> allocated, but not for blocks which haven't been allocated yet.  This
> can be surprising for users; and indeed, for many workloads where you
> aren't using berk_db some other database, all of the files written will 
> be newly created files (or files which are getting rewritten after
> opening with O_TRUNC), so there won't be any difference between
> data=writeback and data=ordered.


That meaning of data=ordered is to ensure that we don't update inode
i_size without writing the data blocks within i_size. So even with
delayed allocation if we have i_size update ( this happen when we
allocate blocks ) we would write the data blocks first.

With that interpretation having a zero block file on crash is fine. But
we should not find the files corrupted.(ie, files with wrong contents).

> 
> So I wonder if we should either:
> 
> (a) make data=ordered force block allocation and writeback --- which
>     should just be a matter of disabling the
>     redirty_page_for_writepage() code path in ext4_da_writepage()


We can't do that because we cannot do block allocation there. So we need
to redirty the page that have unmapped buffer_heads.

> 
> (b) add a new mount option, call it data=delalloc-ordered which is (a)
> 
> (c) change the default mount option to be data=writeback


This won't guarantee that i_size/metadata get updated ONLY after data blocks
are written.

> 
> (d) Do (b) and make it the default
> 
> (e) Keep things the way they are
> 
> Thoughts, comments?   My personal favorite is (b).   This allows users
> who want something that works functionally much more like ext3 to get
> that, while giving us the current speed advantages of a more aggressive
> delayed allocation.
> 
> 						- Ted
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